I could see that as being potentially unbalancing. If it required time and resources (Units to pillage) then it would be akin to using your forces to generate resources. Also the new units (Horde or Hordling as I would name it) should be unable to pillage gaining units, but still be very good at pillaging (as most men who would join the horde would be to pillage, as that is the attraction).

Also throw in a racial chance for an uprising to occur in provinces, or increased chance of uprising, then it would suit it well.